The present invention relates to pressing apparatus for fiber webs.
In particular, the present invention relates to apparatus having an extended press zone for the press treatment of a fiber web of the type which includes a press roll or the like, preferably provided with a hollow face, a first press fabric loop in which the press roll is situated, a press shoe arrangement which acts against or in opposition to the press roll which is supported on a transverse beam or the like, a press belt loop in which the press shoe arrangement is situated, wherein the press roll and press shoe arrangement together form a press zone through which the web to be pressed is guided to run. The press shoe arrangement includes a pressure chamber which is isolated from the external environment by sealing members and into which a pressure medium, such as fluid under pressure, is passed. The pressure chamber produces a pressure over a zone of action within the press zone on the press belt.
The invention also relates to an apparatus of the type described above in which stationary glide elements, which may be provided with a pressure chamber arrangement, essentially replace the press roll.
The commonest arrangement for dewatering fiber webs and, in particular, paper or cardboard webs, comprises conducting the web through a press nip defined by two rolls in opposed relationship with each other. Preferably, one or two press fabrics are conducted through the dewatering press nip in order to carry away the water pressed from the web and to also function to transport the web from the press nip.
As the production rates of paper machines increase, the dewatering of the web in the press section has become a major limiting factor with regard to production rates due to the fact that the press nips defined by a pair of rolls have a relatively short region of action and, therefore, the time spent by the web in such press nips is relatively short at high web speeds. A certain minimum time, however, is required in view of the flow resistance presented by the fiber structure of the web in order for the water to escape from the web and enter into the recessed surface of the press roll or rolls or the press fabric.
It is known to provide a plurality of successive press nips. For example, so-called compact press sections, such as the so-called "Sym-press" press section manufactured by Valmet Oy of Finland, or press sections comprising several separate successive press nips have been used in conventional press sections. However, press sections incorporating several nip presses require large spaces, especially where separate successive press nips are used. A compact press section of the type mentioned above presents difficulties with respect to the optimal placement of their various components as well as in their operation, e.g., with respect to carrying off paper broke. Moreover, expensive suction rolls are commonly employed in nip presses, such suction rolls having large energy requirements in order to produce a vacuum. Moreover, since a perforated shell is a necessity for such suction rolls, strength problems also arise in connection with their use.
Further, a web can only tolerate a certain maximum nip pressure so that an increase in dewatering through increasing the nip pressures is limited by the pressures which the web can tolerate.
Attempts to lengthen the press zone of the press nips through the use of larger diameter rolls and/or soft press fabrics have not proven entirely satisfactory for economic reasons, among others.
However, in view of the various dewatering considerations discussed above, and for other reasons, so-called long or extended nip presses have recently been designed. For example, such long or extended nip presses are disclosed in U.S. Pat. Nos. 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624 and 4,229,253, and British application No. 20 57 027.
Further regarding the state of the art, reference is made to Finnish patent application No. 3554/72 and U.S. Pat. No. 3,783,097. A paper machine press arrangement is disclosed in the Finnish patent application in which a paper web is dewatered in a long press zone which is provided by means of appropriately tensioning flexible belts. However, this arrangement has the drawback that the mechanical strength of the press belts and their associated guide rolls impose a limit on the pressure that can be exerted on the web and, therefore, impose a limit on the extent of web dewatering which can be accomplished. An extended nip press is disclosed in the above-mentioned U.S. patent wherein a plurality of successive press shoes are urged towards a belt and opposing press roll. However, this arrangement has the drawback that the friction between the press shoes and the belt results in the consumption of large amounts of energy. Additionally, the belt and pressure shoes are subject to considerable attrition as a result of friction between them.
A press section incorporating a long or extended nip is disclosed in the above-mentioned U.S. Pat. No. 3,840,429 wherein the web being pressed runs rectilinearly through the press zone defined by a pair of opposed press shoes while interposed between a pair of felts. Pressure is produced in the press zone by means of a pressurized fluid. Belts are provided within the loops of the felts which bound the press zones and transmit the pressure of the pressurized fluid to the web. This extended nip press is not entirely satisfactory in view of sealing problems which are encountered in the press zone. Another drawback of this arrangement is that the web is instantaneously subjected to the total and necessarily high pressure in the nip. However, since the web has a relatively low dry matter content, it cannot tolerate pressures which exceed a certain maximum without breaking. For this reason, the nip pressures must be maintained at a relatively low level. Moreover, it is generally not advisable to subject a web to a high nip pressure which rises abruptly at the very beginning of the pressing operation.